Sulfonated polystyrene-divinylbenzene (PS/DVB) resins have been commercially available for many years and used as cation exchange resin in many different applications such as in water treatment, in recovery of metals from aqueous solutions, as catalysts, in chromatography etc. However, their applicability has been restricted due to their low thermal stability in water or water-containing media. Above 150.degree. C. the aromatic sulfonic acid groups are increasingly hydrolyzed.
A class of sulfonated PS/DVB resins less susceptible to thermal degradation are the sulfoalkylated PS/DVB resins, which have been disclosed in Makromolekulare Chemie, Vol. 184, pp. 1585-1596 (1983). Also disclosed was evidence of their thermal stability, established by measuring the cation exchange capacity of these resins after heating aqueous suspensions of these products in an autoclave at 200.degree. C. Whereas with a conventional sulfonated PS/DVB resin, 75% of the sulfonic acid groups had disappeared after 48 hours, with the sulfomethylated and sulfoethylated resins the loss of sulfonic acid groups was only 30-45% and 20% respectively. Although it was reported that problems had been experienced with the synthesis of the corresponding sulfopropylated resins, even with these resins the loss of sulfonic acid groups was never more than 60% under the above mentioned conditions.
Although these sulfoalkylated resins were found to be more stable at elevated temperatures compared with the conventional sulfonated PS/DVB resins, there is room for improvement, especially when they are used as catalysts in processes which are conducted at high temperatures for long periods of time. Under those circumstances catalyst instability, i.e. premature loss of sulfonic acid groups, would be unacceptable.
Surprisingly a novel class of sulfoalkylated resins has now been synthesized, having a thermal stability--by which we mean stability of the resin in the acid form at elevated temperatures in the presence of water or in aqueous media, hereinafter referred to as thermal stability--which surpasses that of the sulfoalkylated resins previously described. These novel resins are characterized in that the alkyl group carries, in addition to the sulfonic acid group, at least one carboxy group.